Invasive Fly Targets North American Onions, Leeks, Related Crops

The allium leafminer (Phytomyza gymnostoma) damages crops such as onions, garlic, shallots, and leeks via egg-laying in plant tissue and, to a greater degree, through larval feeding on foliage. Native to Europe, the invasive species was first discovered in North America in December 2015 in Lancaster County, Pennsylvania. (Image originally published in Barringer et al 2018, Journal of Integrated Pest Management)

While a certain other invasive insect is grabbing headlines in the mid-Atlantic region of the United States, a more recent invader has also emerged in Pennsylvania and neighboring states and threatens allium crops (such as onions, garlic, shallots, and leeks) in the region.

The allium leafminer (Phytomyza gymnostoma) was first discovered in the United States in Lancaster County, Pennsylvania, in December 2015, and has since spread to at least New Jersey and New York. To alert both the public and the agricultural community about the new pest, researchers at Penn State University and the Pennsylvania Department of Agriculture report P. gymnostoma‘s discovery and outline its biology and management options in a new profile in the open-access Journal of Integrated Pest Management.

Native to Europe, the allium leafminer is a species of fly whose larvae feed on allium foliage and pupate between leaves, in the bulb, or in adjacent soil. Adults also feed on allium leaves and puncture plant tissue to deposit their eggs. Affected plants exhibit stunted growth and wilting. Penn State entomology professor Shelby Fleischer, Ph.D., a co-author on the report, says the allium leafminer can be hard to detect.

“It appears that the adult life stage is very short. So far, in a lab, adults only live for a few weeks at most,” he says. “Also, the larvae undergo a summer aestivation, which is very unusual, but it enables adult activity to be synchronized to the time of growth of various allium species in wild settings. This also means that a large fraction of their life cycle is within plant tissue, and management will need to consider that.”

Fleischer admits that management methods haven’t been tested thoroughly yet in the United States, given the species’ recent arrival, but practices in Europe are informative. Row covers or insecticides timed with P. gymnostoma‘s spring and fall adult flight periods can be effective, and clearing a farmscape of alliums (both crops and wild) when not in season can be an option to reduce host availability for the adults during spring and fall—but the latter may not be an option in areas where alliums are grown continuously or are common in nearby wild areas.

“We also have anecdotal evidence of strong variation in infestation rates among different allium species or growth stages, suggesting ovipositional choice or variation in survivorship,” says Fleischer. “As we learn more about this, it could become a basis for trap cropping as a management tool. Biocontol also needs to be investigated.”

Feeding by the allium leafminer (Phytomyza gymnostoma) results in stunted growth and wilting in allium crops. (Image originally published in Barringer et al 2018, Journal of Integrated Pest Management)

The allium leafminer (Phytomyza gymnostoma) damages allium crops both by egg-laying (dotted oviposition scars) and tunneling in the plants’ leaves. (Image originally published in Barringer et al 2018, Journal of Integrated Pest Management)

Larvae of the allium leafminer (Phytomyza gymnostoma) can feed between leaf layers or in the plant bulb (as shown here), and they often overwinter as pupae between leaf layers as well. This larval behavior combined with a short adult lifespan (three to four weeks) can make the pest hard to detect. (Image originally published in Barringer et al 2018, Journal of Integrated Pest Management)

Further research will illuminate more detailed integrated pest management plans for allium leafminer, but, for now, growers in the U.S. should keep watch for the pest as it likely expands its range, says Lawrence Barringer, entomologist at the Pennsylvania Department of Agriculture and lead author on the profile.

“The allium leafminer is poised to spread across most of North America, overlapping with many allium growing regions in the U.S.,” he says. “Climatically, it is adapted for all but the coldest northern regions and the extremely warm south, based on climate models and its range in Europe. Between natural spread through wild alliums and transport through allium commodities between states, the allium leafminer has the potential to leap frog across the country as a hitchhiker very quickly.”